The roles of metalloproteinase-8 (MMP-8) have not been determined in acute lung injury (ALI). PMN, the predominant inflammatory cell in the lung during the acute-exudative phase of ALI contain MMP-8 within their granules, and release it as a soluble proteinase when they degranulate. Soluble MMP-8 is thought to mediate the activities of the enzyme. However, our data show that activated PMN express a novel form of MMP-8 on their cell surface, in vitro and during ALl in mice. Membrane-bound MMP-8 has similar catalytic activity as soluble MMP-8, but unlike soluble MMP-8, membrane-bound MMP-8 is resistant to inhibition by tissue inhibitors of MMPs (TIMPs). This indicates that it could be an important bioactive form of the proteinase in vivo. Novel data indicate that, surprisingly, TIMP-1 an inhibitor of MMP-8 is also expressed on the surface of PMN where it functions as the receptor for active MMP-8 on the PMN surface. To assess the roles of MMP-8 in ALI, we have generated mice deficient in MMP-8 by gene targeting (MMP-8 -/- mice) and studied them in murine models having features of the acute-exudative and fibro-proliferative phases of ALI. MMP-8-/- mice have increased influx of PMN into the lungs in the acute-exudative phase of ALl compared to wild type mice. However, MMP-8-/- mice are protected from progression to fibro-proliferation in the sub-acute phase. This indicates that MMP-8 has an unexpected anti-inflammatory role in the acute-exudative phase of ALI, and a counterintuitive, pro-fibrotic role in the sub-acute phase of ALl. Our central hypothesis is that PMN-derived MMP-8 plays distinct roles in different phases of ALI As a corollary to this, we will also examine the hypothesis that membrane-bound MMP-8 on PMN is a bioactive form of the proteinase which may contribute in important ways to its activities in vivo. To test our hypotheses, we propose three Specific Aims: Specific Aim 1: Test the hypothesis that on the PMN surface, MMP-8 and TIMP-1 form complexes mediated by the interaction of the COOH-terminals of both molecules, and the NH2-terminal inhibitory domain of TIMP-1 anchors the complex to the PMN surface. Specific Aim 2: Test the hypothesis that MMP-8 down-regulates acute lung inflammation during ALl by inactivating PMN chemokines. Specific Aim 3: Test the hypothesis that MMP-8 promotes progression to fibro-proliferation in ALl by: A) downregulating the lung inflammatory response to injury; and or B) proteolytically regulating the biologic activities of pro-, and/or anti-fibrotic mediators. We anticipate that these studies will provide novel information about the biologic activities of MMP-8 in ALl, and the mechanisms by which MMP-8 retains its activity in vivo. The ALl will be the first lung disease in which MMP-8's activities will be studied. We anticipate that in the future, our studies will facilitate the development of new treatment strategies that may reduce mortality and morbidity in ALl syndromes.